# Newton's Universal Law of Gravitation doubt

The Universal Law of Gravitation states that the module of the force, $$F$$ is $$F = \frac{GmM}{r^2},$$

where $$m$$ and $$M$$ are the mass of the two objects and $$r$$ is the distance between the two objects. From this, it is also derived that the gravitational potential energy is equal to $$E = -\frac{GmM}{r}$$

So does this mean that when the two objects are together, i.e. $$r=0$$, the force and the gravitational energy is infinite? How is it possible that energy and force be infinite?

Please can you explain my doubt?

• radius is calculated from the center of mass, two physical objects cannot have touching centers of mass Aug 6 '19 at 14:09
• @AdrianHoward: Of course they can, just imagine two interlocked C sized objects with masses at the ends. The thing is that OP's formulas are only valid for spherical radially-symmetrical objects. Aug 6 '19 at 20:06
• @Rodrigo: of course, I stated it incorrectly, I was thinking orbs and stated objects. Thanks Aug 7 '19 at 0:08

At $$r=0$$, the force is $$+\infty$$, and the potential energy is $$-\infty$$. This arises if you consider point masses, which we model mathematically using Dirac-delta distributions. Physically, we cannot have two point masses overlapping ($$r=0$$) since this would require an infinite amount of energy. Why? Well, fix one mass, and bring the other mass from $$r=\infty$$ towards $$r=0$$. Then, as the masses get closer, more and more energy is required to bridge the gap. So there is no contradiction, because you can never construct any physical system with overlapping point masses. Furthermore, in any real system, the objects are not point masses, but have some finite volume. Even if you consider an electron as a point mass, the electromagnetic force would far outweigh the gravitational force.